Sulfur containing gases, in general, contain sulfur in the form of hydrogen sulfide contaminated by variable quantities of mercaptans. These gases are generally treated in desulfurization units that operate either with "physical solvents" or with "chemical solvents". The "physical solvents" which are used in processes such as the SELEXOL, RECTISOL, PURISOL and SULFINOL processes absorb both hydrogen sulfide and mercaptans and desulfurization of the gas is complete in one zone.
In units that operate with "chemical solvents" such as monoethanolamine, diethanolamine or methyldiethanolamine, salts are only formed with the acid components of the gas, i.e, the hydrogen sulfide and the carbon dioxide, while essentially all of the mercaptans remain in the treated gas.
If the gas containing the mercaptans also contains higher hydrocarbons, the recovery of which is economically justified, the gas is subjected to an oil stripping treatment. This treatment comprises washing the gas with an oil at very low temperature (lower or equal to about -30.degree. C.). The oil simultaneously absorbs the mercaptans and the higher hydrocarbons. The mercaptans and higher hydrocarbons are separated from the absorption oil and removed. The mercaptans are generally recovered in the hydrocarbon fraction having a boiling point corresponding to the boiling point of the mercaptans.
In gases for commercial use, the total quantity of sulfur must be lower than 50 mg/m.sup.3, and for the gases to be used in reforming units, the specifications are even more strict. For the majority of sulfur containing gases treated in desulfurization units using "chemical solvents" (amines), which are not subsequently submitted to an oil stripping treatment, a supplementary purification is necessary.